Method for preventing slag formation in



Sept. 10, 1963 w. A. WALLS ETAL METHOD FOR PREVENTING SL AG FORMATION INHIGH TEMPERATURE BOILERS Filed Dec. 29, 1960 mu; 2 wou m 96 8.6; 2233 7mm 3 w vlmv E5: N. mu v23 5 Szfid w r o 2 m v.23 52:25 515 52% Ommonnzmkzmu William AWGlIs William S. Procror ATTORNEY United StatesPatent 3 103 486 METHOD FOR PREVIlNTlNG SLAG FORMATKGN 1N HlGHTEMPERATURE BQHJERS 'William A. Walls, Drexel Hill, and William S.Proctor,

This invention relates to a method for preventing slag formation andimproving the fuel combustion characteristics in high temperatureboilers fired with heavy fuel oils and, more particularly, to theremoval of sodium Compounds fromfuel oils such as residual oilscontaining sodium compounds and vanadium compounds as impurities torender such fuels suitable for use in high temperature boilers.

During recent years there has been a trend toward the use of hightemperature, oil fired boilers, particularly high temperature marineboilers in which superheateroutlet temperatures of 850 F. up to as highas 1020 F. or higher are employed. The fuels normally used are of theresidual type, such as bunker C, which contain various metalliccompounds as impurities. In particular, these fuels contain sodiumcompounds predominantly the chloride and vanadium compounds probably inthe form of complex organo-vanadium compounds. When such fuels areburned in thesehigh temperature boilers, serious slagging occurs on thetubes and adjacent surfaces. It is believed that slagging arises from acomplex combination of sodium and vanadium compounds, some of which haverelatively low fusion temperatures. Thus, in high temperature boilersthese compounds deposit rapidly as a semi-molten mass which adheres tothe tube surfaces until bridg'ng occurs between the tubes whicheventually leads to plugged air passages through the boiler. Thegeometry of marine boilers resulting from the compactness dictated byshipboard space limitations requires that the tube spacing of theboilers be small which permits particularly quick slag bridging and alsointerferes with soot-blower effectiveness.

The slagging has to be removed by cooling the boilers and manuallychipping off the slag which operation frequently results in serious tubeand header damage. This slagging problem is so serious that frequentlyit has been found necessary to operate the boilers well below theirnormal high temperature design operating temperatures. In addition, ithas been found necessary to avoid bunkering at certain ports because ofprevious bad slagging experience with the fuels available at such ports.Other efforts to solve the slagging problem have involved the use ofadditives to the fuel. None of these measures has proved to besuccessful, thus, for example, when the boilers were operated attemperatures below their design temperatures they obviously wereoperating at a less efficient level. Moreovenwhen it became necessary toavoid bunkering at certain ports, inefficient scheduling resulted withattendant increased costs and, finally, additives did not alleviate theslagging problem.

It now has been found that if the sodium content of a fuel oilcontaining both sodium compounds and vanadium compounds is reduced by atleast about 65 percent to 70 percent by weight based on the originalweight of the sodium in the oil, slaggin-g in high temperature boilersis substantially completely eliminated and the fuel combustioncharacteristics are greatly improved.

It is an object of this invention to provide a method for preventingslag formation and for improving the fuel combustion characteristics inhigh temperature boilers fired with fuel oils containing sodiumcompounds and vanadium compounds as impurities.

It is another object of this invention to provide a 3-,lh3,48 PatentedSept. 10, 1963 "ice method for reducing the sodium content of a fuel oilcontaining both sodium compounds and vanadium compounds by at leastabout 65 percent to 70 percentby weight based on the original weight ofthe sodium in the oil.

It is another object of this invention to provide a method forpreventing slag formation and for improving the fuel combustioncharacteristics in high temperature boilersfired with fuel oilscontaining both'sodium cornpounds and vanadium compounds by reducing thesodium content of the fuel oil by at least from 65 percent to 70 percentby weight, based on the original Weight of the sodium in the oil,utilizing water extraction for reducing the sodium content.

Other objects of this invention will be apparent from the descriptionand claims that follow.

In accordance with this invention, heavy fuel oil of the residual type,such as bunker C fuel oil, is admixed with water for a suflicient timeto extract at least about 65 percent to 70 percent by weight of thesodium compounds based on the original Weight of the sodium compounds inthe oil and thereafter separating the water from the oil bycentrifugation.

In order for a more complete understanding of the invention, referenceis hereby made to the accompanying drawing which is a diagrammatic viewof an apparatus embodying the invention.

With reference to the drawing, numeral 1 indicates a pipe originating ata storage tank for feeding the raw oil, bunker C fuel oil for example,to the intake side of raw oil feed pump 2 from which it is pumpedthrough pipe 3 into pipe '4 which is surrounded by a heating jacket 5where the oil is heated to a temperature of from 190 F. to 210 F. Ademulsifier or emulsion breaker, for example the demulsifier knowncommercially as Tretolite, contained in tank 6 is added through pipe 7by demulsifier feed metering pump 8 through pipe 9 to the raw oilcontained in pipe 1. The heated oil after leaving jacket 5 is passedthrough pipe 10 into mixer 11 where it is thoroughly mixed with water byagitator 12.

Fresh water from storage is furnished to the system through pipe :13 tothe intake side of fresh water feed metering pump 14 from which'it ispumped into pipe 15 which is surrounded by heating jacket '16 Where thewater is preferably heated to a temperature of from 180 F. to 210 F. Theheated water is passed through pipe 17 into mixer 11 wherein it isthoroughly mixed with the oil as has been described. The mixture of oiland water is passed from mixer 11 through pipe 18 through a strainer orfilter 19 where any large lumps or particles are removed The oil andwater mixture is then passed requires that the through pipe 249 intocentrifuge '21 \where it is separated 7 into an oil phase and an aqueousphase which latter phase contains sludge particles as well as theextracted sodium salts.

The aqueous sludge phase leaves the centrifuge through pipe 22 and isdelivered into settling tank 23. This tank which is of conventionaldesign fitted with conventional dams or weirs permits the sludge tosettle and allows the sludge together with Waste water to be dischargedfrom the tank through pipe 24 to the intake side of pump 25 whichdischarges the separated water and sludge through pipe 26 to waste. Ifcentrifuge 21 is of the type which final oil-Water separation be carriedout in a settling tank, oil may be recycled to the centrifuge by meansof pipe 27 through recycle pump 28 and pipe 29. If the centrifuge 21 isof the type in which the oil- Water. interface is maintained within thecentrifuge itself, water is recycled through pipe 27, pump 28, pipe 29into centrifuge 21 Where it supplies the necessary quantity of water tomaintain the seal in the centrifuge. The net waste water together withthe sludge is then discarded through pipe 24, pump 25, and pipe 26 ashas been described.

The cleaned fuel oil is ejected from the centrifuge through pipe 30 intocleaned fuel oil tank 31 from where it is pumped through pipe 32 by pump33 into pipe 34 which is cooled by jacket 35 to lower the temperature ofthe oil to approximately 120 F. and thereafter the cooled oil istransferred by pipe 36 to the cleaned oil storage.

This invention is particularly applicable to fuels of the residual typecontaining a high level of sodium compounds and vanadium compounds. Suchlevels comprise from 30 parts per million to 100 parts per million orhigher of sodium and from 50 parts per million to 500 parts per millionor higher of vanadium. In general, the average for such fuels is of theorder of 60 parts per million of sodium and 270 parts per million ofvanadium, i.e. a sodium to vanadium ratio of approximately 0.22.

When such higher metal content fuels are burned in high temperatureboilers, i.e. a boiler operating at 850 F. or higher, serious slagproblems are encountered as has been described. If, however, inaccordance with this invention the sodium content of such fuels isreduced to approximately 20 parts per million, i.e., reduced from 65percent to 70 percent of the original sodium content, slagging issubstantially completely eliminated. This reduction in sodium contentreduces the average sodium to vanadium ratio to a value of 0.074.

The instant invention, however, is not applicable to fuels having aspecific gravity greater than 0.99 at 60 F., i.e. not less than 11.5 APIsince, with fuels having a specific gravity greater than this limit, thegravity differential between the extraction water and the fuel is notsufficiently large to permit clean separation by the centrifuge or bygravity settling.

The type and quantity of demulsifier employed is not a critical featureof this invention. Numerous compounds are available commercially forbreaking oil and Water emulsions with the preferred dosages beingspecified for each compound by the manufacturer. With the aforementionedTretolite compound a dosage of 0.6 cubic centimeter per gallon of fuelwas found to be completely suitable in aiding the separation of waterand oil in the centrifuge.

The oil after being admixed with the demulsifier is heated to atemperature of from 190 F. to 210 F. and thereafter is admixed withwater which has been preheated to approximately the same temperature ofthe oil.

The heating of the oil serves two purposes. It reduces the viscosity ofthe oil so that it may be more easily admixed with the water and itpermits the use of water at equally high temperatures under whichconditions the sodium salts are more soluble and therefore more easilyextracted from the oil by the water.

The quantity of water utilized may range from 3 volume percent to 15volume percent based on the volume of the oil. In general, it has beenfound that volume percent of water provides the optimum treat forremoval of the desired quantities of sodium. The mixing of the oil andwater is carried out in a conventional mixer equipped with a high speedpropellor and, if desired, internal baffles. The mixer is sized so thatthe hold up time ranges from 2 to 3 minutes although the time of mixingobviously will be a function of the efiiciency of agitation, theviscosity of the oil and other factors. Such mixing should be suflicientto provide efficient contacting of the oil with the water so that thedesired quantities of sodium compounds are extracted from the oil. Theseengineering principles are well understood and therefore need not befurther elucidated here.

The mixture of oil and water is pumped from the mixer through aconventional strainer or filter which is provided to remove any largeparticles of solid material which may be found in the mixer and whichwould subsequently interfere with the operation of the centrifuge. Thestrained oil-water mixture is pumped into a conventional high speedcentrifuge, for example, the self-cleaning type unit provided withinternal reflux of water which permits passage of solid sludge and dirtparticles from the peripheral nozzles while maintaining the oil-waterinterface within the machine. The position of the interface in such acentrifuge may be controlled by interchangeable dams which are changedwhen fuels of different gravities are used successively. The interfacealso may be adjusted with the machine in operation by raising orlowering the temperature of the recycle water. The source of suchrecycle water will be described hereinafter. The exact type ofcentrifuge which is employed for separating the oil and water mixture isnot a critical feature of this in vention and one conventional type hasbeen described but other commercially available machines are equallysuitable. In addition to containing the extracted water soluble salts,the water phase contains some heavy sludge particles. The combined waterand sludge leaving the centrifuge is introduced into a settling tank. Inthe settling tank the sludge settles to the bottom and may be removedalong with the net waste water. The purified water from which the sludgehas settled is recycled to the centrifuge in amounts sufficient tomaintain the centrifuge seal. This quantity is normally of the order ofthe oil being separated from the oil-water mixture. The quantity ofrecycle is controlled by the dynamic balance of the machine such thatonly enough recycle water is accepted to satisfy the demand of themachine with excess recycle water being bypassed to the settling tank.The extracted fuel oil after being ejected from the centrifuge is pumpedthrough a cooler and then to storage. It is generally necessary to coolthe oil to approximately 120 F. since the pumps which operate to feedthe boilers cannot handle but oil of the order of 180 F. which is thetemperature of the oil discharged from the centrifuge.

In order to demonstrate that the instant invention substantiallycompletely eliminated slag formation and improved the combustioncharacteristics in a high temperature boiler fired with conventionalhigh sodium and high vanadium bunker C fuels, an apparatus similar tothat described in the drawing was installed in a modern supertanker ofapproximately 30,000 tons deadweight capacity at summer load draft. Thistanker was equipped with two Combustion Engineering SM sectional headertype marine boilers which operated at a total steam temperature of 1020F. This tanker operated on conventional bunker C type fuel.

Bunker C fuel or No. 6 fuel as it is also designated is the highviscosity bottoms fraction of crude oil. In general, it is specifiedthat such fuel has a maximum Furol viscosity of 300 seconds at 122 F.and a minimum Furol viscosity of 45 seconds at 122 F. The specificationsfor the fuel burned in this tanker, however, was a Furol viscosity of200 seconds at 122 F. maximum and seconds at 122 F. minimum.

The tanker had a history of serious slag deposition on the boiler tubesand consequently high tube renewal requirements. .Prior to beginning thetest run on the low sodium fuel experimental program, the boilers werecleaned completely and tube renewals made where necessary. Afterinstallation of the apparatus described in the drawing, the tanker wasoperated in a normal manner for several months on bunker C fuels whichvaried in gravity from about API to 15.8 API, Furol viscosities rangingfrom 102 seconds at 122 F. to 204 seconds at 122 F., sodium contentsranging from 29 parts per million by weigh-t to 96 parts per million byweight (average, 60 parts per million) and vanadium contents rangingfrom 27 parts per million by Weight to 539 parts per million by weight(average, 270 parts per million).

The treating equipment was designed to process up to 20 gallons perminute of bunker fuel and consequently since the ship burned only about580 barrels per day of fuel the equipment built up an inventory oftreated fuel during each voyage so that the ship was always operated ontreated fuel. Treating temperatures of approximately 200 F. wereutilized together with a 5 volume percent treat of fresh water. It wasfound that the sodium content of the fuel was reduced from 65 percent.to 70 percent or higher with the 70 percent level being maintained morefrequently toward the end of the experimental period as experience wasgained in operating the equipment. The vanadium content of the fuel wasreduced by only about 16 percent maximum indicating that, \as was knownheretofore, the vanadium compounds are not present predominantly inwater soluble form. After approximately 11 months of operation utilizingtreated fuel the boilers were inspected and found to be free of slagdeposits and with only a very thin layer of exceedingly brittle depositwhich was so friable, thin, and loosely adhering that it could beremoved by the fingers.

During the time treated fuel was utilized it was found that thecombustion characteristics of the fuel had been improved sinceimpingement against the rear wall of the furnace, drooling at theburner, oil on the furnace floor and collection of oil in thesuperheater tanks disappeared. Along with these changes refractorytemperatures dropped which prolonged the life of the brickwork. Inaddition, flame photographs made at the time untreated fuel was beingused compared with photographs made when the treated fuel was being usedshowed that a superior flame pattern was obtained with the treated fuel.

These full-scale experiments demonstate that the method of the instantinvention prevents slag formation in high temperature oil-fired boilersand also improves combustion characteristics of the fuel in suchboilers.

We claim:

1. A method for preventing slag formation and improving the fuelcombustion characteristics in high temperature boilers fired with heavyfuel oils having a specific gravity not greater than 0.99 at 60 F. andcontaining sodium compounds and vanadium compounds as impurities, whichcomprises adding a demulsifier and from 3 volume percent to 15 volumepercent of fresh Water to the impuritycont-aining oil, mixing theimpurity-containing oil, demulsifier, and water at a temperature of from190 to 210 F., whereby a two-phase mixture is obtained in which thewater soluble sodium compounds are extracted by the water phase and thesodium content of the :oil phase is reduced by at least 65 percent to 70percent, and centrifugally separating the aqueous phase from the oilphase.

2. A method for preventing slag formation and improving the fuelcombustion chamacteristics in high temperature b oilers fired with heavyfuel oils having a specific gravity not greater than 0.99 at 60 F. andcontaining sodium compounds and vanadium compounds as impurities, whichcomprises adding a demulsifier and about 5 volume percent of fresh Waterto the impurity-containing oil, mixing the impurity-containing oil,demulsifier, and Water at a temperature of from 190 F. to 210 F, wherebya two-phase mixture is obtained in which the water soluble sodiumcompounds are extracted by the water phase and the sodium content of theoil phase is reduced by at least 65 percent to 70 percent, andcentrifugally separating the aqueous phase from the oil phase.

3. A method for preventing slag formation and improving the fuelcombustion characteristics in high temperature boilers fired with heavyfuel oils having a specific gravity not greater than 0.99 at F. andcontaining sodium compounds and vanadium compounds as impurities, whichcomprises adding a demulsifier and about 5 volume percent of fresh waterto the impuritycontaining oil, mixing the impurity-containing oil,demulsifier, and water at a temperature of about 200 F., whereby atwo-phase mixture is obtained in which the water soluble sodiumcompounds are extracted by the water phase and the sodium content of theoil phase is reduced by at least percent to percent, and centrifugallyseparating the aqueous phase from the oil phase.

4. A method for preventing slag formation and improving the fuelcombustion characteristics in high temperature boilers fired with heavyfuel oils having a specific gravity not greater than 0.99 at 60 F. andcontaining sodium compounds and vanadium compounds as impurities, whichcomprises adding a demulsifier and from 3 volume percent to 15 volumepercent of fresh water to the impurity-containing oil, mixing theimpurity-containing oil, demulsifier, and Water at a temperature of from190 F. to 210 F., whereby a two-phase mixture is obtained in which thewater soluble sodium compounds are extracted by the water phase and thesodium content of the oil phase is reduced by at least 65 percent to 70percent, centrifugally separating the aqueous phase from the oil phase,and cooling the separated oil phase to a temperature of about F.

References Cited in the file of this patent UNITED STATES PATENTS2,682,496 Richardson et a1 June 29, 1954 2,728,714 Winkler et a1. Dec.27, 1955 2,774,722 Burkland et al. Dec. 18, 1956 2,785,120 Metcalf Mar.12, 1957 2,789,083 Hardy Apr. 16, 1957 2,825,678 Iahn'ig et al. Mar. 4,1958

1. A METHOD FOR PREVENTING SLAG FORMATION AND IMPROVING THE FUELCOMBUSTION CHARACTERISTICS IN HIGH TEMPERATURE BOILERS FIRED WITH HEAVYFUEL OILS HAVING A SPECIFIC GRAVITY NOT GREATER THAN 0.99 AT 60*F. ANDCONTAINING SODIUM COMPOUNDS AND VANADIUM COMOOUNDS AS IMPURITIES, WHICHCOMPRISES ADDING A DEMULSIFIER AND FROM 3 VOLUME PERCENT TO 15 VOLUMEPERCENT OF FRESH WATER TO THE IMPURITYCONTAINING OIL, MIXING THEIMPURITY-CONTAINING OIL, DEMULSIFIER, AND WATER AT A TEMPERATURE OF FROM190* TO 210* F., WHEREBY A TWO-PHASE MIXTURE IS OBTAINED IN WHICH THEWATER SOLUBLE SODIUM COMPOUNDS ARE EXTRACTED BY THE WATER PHASE AND THESODIUM CONTENT OF THE OIL PHASE IS REDUCED BY AT LEAST 65 PERCENT TO 70PERCENT, AND CENTRIFUGALLY SEPARATING THE AQUEOUS PHASE FROM THE OILPHASE.